[The "project design flood" is the maximum flood that the Army Corps of Engineers has engineered the Mississippi River's flood control structures to accommodate; the image here (via America's Wetland and Loyola University) shows those flows in cubic feet per second.
I've been slow to link (though, as promised, the flood blogging is going to pick back up now) to Brett Milligan's great post on how the 1927 floods produced a massive shift in the strategies used to control the Mississippi River, and how we tend to forget those changes; at any rate, the post says the following in discussing the image above:
"The strategies generated by the the floods of 1927 seem to have been pretty close to a paradigm shift for the Army Corps of Engineers’ then current river control strategies. There was a hard realization that all those miles of levees that worked at a regional scale were impossibly small play things when it came to dealing with above average stormwater drainage funneled from 41% of the land surface of the nation (a definition of an event landscape). Rather than just making the levees taller and thicker (which they did), they also made some well-observed retroactive moves by actually giving way to the river at some of their weakest levees points through the creation of the floodways (the Bonnet Carre Spillway is located where a levee had failed multiple times). By expanding the infrastructure and creating release valves, the river was still under control, but only by giving it back its floodplains when its asks for them.
Like floods themselves, the floodways are stealthy features in the landscape. Covered in crops and green leafy things much of the time, their systemic function and appropriation slips to the background in the calm between the big events. When the floods return, the Army Corps is forced to dust off their presentation boards and attend the requisite media circus to review the instrumental theory designed and codified over 50 years ago, which produced a diagram with such persistent instrumental effect that Deluzians should appreciate."
Read the whole post at Free Association Design.]